It is well known that titanium dioxide absorbs UV radiation and catalyzes the degradation of surrounding organic media. This undesirable photo-catalytic reaction can be arrested before titanium dioxide is used as a white pigment in paints, coatings, and plastics. Photo-irradiation of TiO2 with photon energy greater than the band gap energy excites the electron of the valance band to the conduction band, creating holes in the valence band and high energy electrons in the conduction band. Consequently, following irradiation, the TiO2 acts as either an electron donor or a hole donor to reduce or oxidize the organic compounds present in the surrounding media by forming highly reactive free radicals. These free radicals, if not neutralized or prevented from formation, initiate the degradation of the binder in paint systems, causing chalking and fading. It is known that the photo-activity of TiO2 can be reduced by subjecting it to inorganic post treatment using zirconia, silica, or alumina. The inorganic post treatment also enhances the dispersion and optical properties of the TiO2. It is further known that the precipitation conditions and nature of treatment layers have a significant influence on the durability and optical performance of the treated pigment.
Several examples of zirconia and alumina as components of a post treatment process are described in many patents, but the treatment conditions adopted and the precipitation of a composite layer of zirconia and silica are not disclosed.
Great Britain Patent No. GB 2,108,098 describes the use of a hydrous oxide layer of zirconia as a first layer on TiO2, a second layer of dense amorphous silica, and a final layer of a hydrous oxide of alumina, to improve resistance to photochemical degradation. Both the zirconia and silica layers are precipitated separately and the alumina precipitation is done at alkaline pH.
Another approach is described in U.S. Pat. No. 3,437,502, which describes a highly durable TiO2 pigment made by treating TiO2 with a dense silica layer and an alumina layer precipitated at a pH above 4.5. The pH is set at pH 6 to 8 by adding either sulfuric acid or NaOH.
U.S. Pat. No. 3,981,737 describes an alumina treatment at alkaline pH in the presence of vanadium ions to form light-resistant TiO2. The alumina is added to a pre-set alkaline slurry.
U.S. Pat. No. 4,239,548 describes a light stable/fast TiO2 pigment for paper laminates, in which alumina is precipitated at pH 5 to 7 by adding aluminum sulphate. NaOH is used for setting the pH and other ions like cerium and phosphate are also added.
U.S. Pat. No. 4,450,012 describes coated mixed phase rutile pigments having a first coating of an oxide or hydrated oxide of titanium, zirconium, tin, or a mixture thereof, and a subsequent coating of an oxide or hydroxide of aluminum. The pigments resist flocculation when formulated in lacquers hardened with acid catalysts.
U.S. Pat. No. 4,405,376 describes a titanium dioxide pigment along with a process for producing the pigment, wherein the pigment displays improved durability and dispersibility and comprises a pigmentary titanium dioxide core particle, an inner coating of hydrous oxides of tin and zirconium, and an outer coating of a hydrous oxide of aluminum. NaOH is used for raising the pH.
U.S. Pat. No. 4,447,271 describes a highly durable and weather-resistant pigment made by treating TiO2 with a dense amorphous silica and then with a hydrous oxide of zirconia. Optionally, an outer coating of a hydrous oxide of alumina is provided. The silica precipitation is done at alkaline pH (pH 7.5 to 9.4). The alumina layer is precipitated at pH 10.0 to 10.5.
U.S. Pat. No. 5,203,916 describes a pigmentary titanium composite possessing good durability and excellent optical properties, consisting essentially of a particulate titanium dioxide base, a hydrous zirconium oxide layer deposited on the titanium dioxide base at pH 8, and a hydrous alumina layer deposited on the hydrous zirconium oxide layer. A process for producing such a pigment is also described.
U.S. Pat. No. 5,700,318 describes a durable pigment for plastic applications, and methods of precipitating different forms of alumina at different pH levels. The pH is reduced to 3.5 after adding a first portion of aluminate. Then, the pH is increased to 5.75 by adding NaOH before adding a second portion of aluminate. The pH is then increased to 8 by using NaOH before adding a final portion of aluminate. Neither a silica layer nor a zirconia layer is used in addition to the alumina layer.
U.S. Pat. No. 5,730,796 describes a highly durable pigment in which TiO2 is treated with ceria and silica and then with alumina. An aluminate solution is added at pH 7.5 and the pH is increased to 10.5 after the addition, for precipitation.
U.S. Pat. No. 5,554,216 describes a high gloss, highly durable pigment production method wherein alumina is precipitated in the presence of a very small amount of molybdenum at a pH of from 6 to 10.
U.S. Pat. No. 5,976,237 describes the production of durable TiO2 pigment with good dispersibility and optical properties for use in plastics and paint compositions. Three layers of coatings are described. The first coating is made of silica or alumina, an optional second coating is made of zirconia or tin oxide or ceria or titania, and the third coating is made of alumina. The alumina treatment is conducted at a different pH. Sodium aluminate is added to an acidic slurry and the pH is adjusted to 3.5 and maintained under digestion, then to 5.75 and maintained under digestion, then to 6.5 and maintained under digestion, and finally to 7.0 and maintained under digestion. The pH adjustment at each step is done using NaOH.
U.S. Pat. No. 6,656,261 B2 describes substantially sulfate-free titanium dioxide pigments with improved gloss and/or durability and comprising alumina, zirconia, and optionally, phosphate compounds. Methods of making these pigments are also described. These pigments are useful in the manufacture of paints and plastics. This patent describes a method comprising wet treating titanium dioxide with, first, an alumina compound to form an alumina layer, followed sequentially by wet treatment with a zirconia compound to form a zirconia layer. Nothing is taught as to the expected outcome. The description of an outer zirconia layer is in sharp contrast with the other references described herein, which describe the use of a zirconia inorganic oxide treatment. The deposition of zirconia is carried out as an intermediate step only and a final alumina treatment is applied in order to ensure the pigment's compatibility with modern ingredients used in paints, plastics, and paper, for instance.
U.S. Pat. No. 7,135,065 describes a post treatment of TiO2 to obtain weather-resistant pigment with good optical properties. The pigment is coated sequentially with hydrous tin and zirconium. At least one other component from silicon and titanium is additionally precipitated on the pigment particle surface. Then, a final layer of alumina is precipitated. The post treatment components are added to the aqueous TiO2 suspension either in an acidic pH range (pH 3.0) or in an alkaline pH range (pH 10.0). The pH value is subsequently set to 6.0 to 8.0 before sodium aluminate/aluminum sulphate addition. The method incorporates hydrous SnO2 together with ZrO2, and then a coating of silica or alumina or titania with zirconia, and then a final layer of alumina. The patent describes that SnO2 must be added prior to ZrO2, and then the other chemicals, in sequence, to get the required weather resistance.
U.S. Pat. No. 7,238,231 describes a zirconia treated TiO2 pigment in which the zirconia treatment is done after re-dispersing a washed semisolid filter cake of silica and alumina-treated TiO2 pigment using ammonium zirconyl carbonate. The resultant pigment is then spray dried.
U.S. Pat. No. 8,105,432 describes a method for making high durability and easily dispersed pigment by adding citric acid to stabilise amorphous alumina. The combination of silica and citric acid-stabilized alumina are described as the cause of improved dispersion and durability. The precipitation of silica and alumina is done at various controlled pH levels by using either NaOH or HCl at 95° C.
Despite extensive prior art targeting the improvement in durability of titanium dioxide, either by depositing zirconia or tin and zirconia to titanium dioxide pigments, or by using alumina stabilised by organic acid, further improvements are continually being sought, especially due to the escalating raw material cost of zirconium chemicals. In none of the aforementioned references are the benefits of a lower zirconia content described, let alone that are achieved by treatment with a zirconia-silica composite, to result in the manufacture of a multipurpose pigment. In addition to these benefits, a reduced number of treatment steps and less consumption of acid and base are among the other advantages achieved by the method of the present invention disclosed below.